1. Field of the Invention
The present invention relates to a process for whitening kaolin which permits the obtention of higher indices of whiteness and a reduction in the size of the particles of the kaolin product obtained.
2. Description of Previously Existing Technology
As is widely known in the field, kaolin is the term generally used to designate a material essentially consisting of clay minerals of the kaolinite group (Al.sub.2 O.sub.3.2SiO.sub.2 H.sub.2 O), i.e., a silicate of hydrated aluminum.
Geologically, kaolins appear in the form of an extremely fine powder, resulting from the weathering of feldspathic rocks, the primary economically exploitable deposits of which originated in primary kaolinization in situ of rocks containing high concentrations of feldspathic minerals (primary kaolins), or arising from being borne along by river or lake freshwater currents, with subsequent settling and selection of primary kaolins associated with other minerals such as quartz or mica (secondary kaolins).
Owing to their characteristics of good chemical inertness and fine particle size, in addition to their generally white color, their broad availability and low costs, kaolins are widely used industrially as fillers or coating materials for paper, ceramics, inks, rubbers, plastics and fertilizers, among other substances.
One problem with kaolins, however, is the presence of iron and titanium oxides, which alter their white color--one of their most important characteristics. The content and forms in which the contaminating oxides are present vary according to the origin of the kaolin.
Innumerable solutions have been previously proposed to improve kaolins, aimed at removing the contaminating iron oxide in order to whiten kaolins. Diverse whitening processes using different reagents are described, for instance, in the text "Clays and Clay Minerals--Proceedings of the Seventh National Conference on Clays and Clay Minerals," Washington, D.C., October 1988, pages 317 and 327, the text of which is included herewith as a reference.
Among these well-known processes, the most widely used are leaching the kaolin with an acid solution containing sodium dithionite (Na.sub.2 S.sub.2 O.sub.4) or a mixture of sodium bisulfite and metallic zinc (NaHSO.sub.3 /Zn.sup.o).
For the specific case of Brazilian kaolinitic clays, which are principally darkened by the presence of high concentrations of goethite (FeOOH), however, the use of these aforementioned reagents is not sufficient to provide the whiteness required for certain applications of kaolin.
In fact, laboratory experiments have shown that leaching samples of goethite with acid solutions containing the above-mentioned reagents resulted in dissolving or eliminating from 12.5 to 18.7% of the iron contained in the sample, which is low for obtaining the higher values of whiteness.
Therefore, there exists a need for a process for whitening kaolin, which, by means of dissolving greater amounts of iron in kaolin, will make it possible to obtain greater values of whiteness.